The present invention relates generally to cardiac assist and/or resuscitation systems for restoration or augmentation of natural blood flow and, more particularly, to implantable systems and associated methods for assisting the natural contractions of the heart.
Following a heart attack or as a result of other cardiac disease states, the heart""s ability to pump blood can be seriously impaired. Conventional cardiac assist systems employ a variety of pumping approaches for assisting a failing natural heart. Generally, there are two categories of cardiac assist systems: those which contact blood, referred to herein as blood-contacting cardiac assist systems; and those which do not, referred to herein as non-blood-contacting cardiac assist systems.
A primary drawback of blood-contacting cardiac assist systems is the associated risk of thromboembolism. Although significant efforts have been made to reduce or eliminate this problem, the continued risk of thrombosis has restricted blood-contacting cardiac support systems to temporary or short-term applications. In addition to the risk of thrombosis, blood-contacting cardiac assist devices typically also experience calcification. The degree of calcification increases with time, again making these devices undesirable for long term applications.
Non-blood-contacting cardiac support systems significantly reduce the risk of thromboembolism and calcification. One conventional approach has been to directly apply forces to the heart so as to facilitate pumping. For example, U.S. Pat. No. 4,304,225 to Freeman discloses a non-contacting cardiac assist system designed to compress all or part of the heart by alternately tightening and releasing a circumferential compression band. Another conventional device, described in U.S. Pat. No. 4,583,523 to Kleinke et al., is an articulated mechanical device for applying an encircling force to the aorta. European Publication No. 0583012 A1 to Heilman et al. teaches the application of a similar device to the heart. Still other conventional systems, such as U.S. Pat. No. 4,411,268 to Cox and U.S. Pat. No. 4,813,952 to Khalafalla disclose an approach of encircling the heart with the latissimus dorsi muscle to achieve a desired compression of the heart.
Another class of non-blood-contacting cardiac assist devices uses hydraulic or gas pressure to displace an equivalent volume of blood in the heart through pressure applied to the outer surface of the heart, the epicardium. One conventional approach has been to use a housing of rigid construction for enveloping, at least partially, the ventricular region of the myocardium. The inner surface of the housing typically has a distensible elastic membrane adjacent to the myocardial wall. Pumping fluids are fed to the chambers defined by the housing and the membrane to apply pressure on the myocardial wall. In some instances the outer portion of the housing is formed of a flexible, non-distensible, member with an elastic distensible inner membrane. In general these conventional approaches utilize one or more compartments, each characterized by an elastic inner wall and an inelastic outer wall. Filling the compartments compresses the myocardium of the ventricle to aid pumping. When pumping is facilitated in this manner, a volume of inflating fluid or gas is required to displace an equal volume of blood. Cardiac assist devices of this general class are described in U.S. Pat. Nos. 2,826,193; 3,371,662; 3,455,298, 3,587,567; 3,613,672; 4,048,990; 4,192,293; 4,506,658; 4,536,893; 4,690,134; 4,731,076; 5,119,804; 5,131,905; 5,169,381; and 5,273,518.
Other approaches have employed a concave, gel-filled compression pad activated by a plate on its outer surface (U.S. Pat. Nos. 4,925,443; 5,098,369; 5,348,528), a cardiac assist envelope designed for minimally invasive implantation (U.S. Pat. No. 5,256,132); or a cardiac assist device having a fluid filled jacket encasing at least the heart ventricles to provide a compliant, completely passive support (U.S. Pat. No. 4,957,477).
A drawback to these cardiac assist devices is that they typically pump blood by displacing the blood with an equal inflation volume of a hydraulic fluid. As a result of this limitation, such systems require large reservoirs of the hydraulic fluid and/or complex pumping protocols.
To overcome this and other drawbacks, cardiac assist devices have been devised which displace blood with an inflation volume smaller than the displaced blood volume. Such cardiac assist devices typically produce higher pumping capacities through the injection of a relatively smaller quantity of fluid or gas under high pressure. Generally, these devices utilize a chamber or wrap having a number of inflatable segments.
For example, commonly owned U.S. Pat. No. 5,713,954 to Rosenberg et al. describes a non-blood-contacting cardiac assist device having tubes that contract a circumference of the heart when inflated. In one embodiment, the Rosenberg device is constructed of vertically-oriented, cylindrical (tube-shaped) inflation chambers arranged to form a ring and surrounded by a nondistensible sheath to form an artificial myocardium or heart wrap. The administration of a fluid under pressure causes the tubes of these conventional devices to have an expanded cross section, which is generally circular. When the fluid is withdrawn, the tubes flatten perpendicular to the direction of force generated by the pressure in the heart. When the tubes are deflated, the circumference of the pumping chamber is equivalent to the value of the number of tubes in the wrap multiplied by one half the circumference of one of the constituent tubes. When the tubes are fully inflated, the circumference of the pumping chamber is equivalent to the product of the number of tubes and the inflated diameter of one of the constituent tubes. When the wrap circumference is minimized there is no dilation of the tube circumference.
The resulting contraction of the circumference of the heart wrap is maximally 36%. This limit is due to the geometry of the device and is independent of the radius of the tubes chosen. Therefore, the volume of each tubes can be made small while maintaining a constant ejection volume. However, the work done is, in all cases, the same. The result is that smaller tubes require a higher pressure to attain a circular cross section. In general, for constant work performed, the inflation pressure is inversely proportional to the inflation volume.
U.S. Pat. No. 3,464,322 to Pequigot also discloses an artificial blood pumping chamber that has walls which are formed from an arrangement of inflatable tubes. A drawback of the Pequigot device is that the inflation chamber tubes are free to dilate when inflated. The Rosenberg device overcomes the drawbacks of the Pequigot device since the circumference of the inflation chambers of the Rosenberg device cannot exceed the dimensions of the fabric pockets in which they are imbedded. Consequently, the pumping action resulting from a contraction of the Rosenberg heart wrap is not defeated by dilation of the radii of the inflation chambers. Therefore, the Rosenberg device is more likely to reach the theoretical maximum contraction of 36%. However, like the Pequigot device, the Rosenberg device cannot exceed this limiting maximum contraction ratio.
One drawback with the above blood pumping devices is that the actual extent of contraction, expressed as a percentage of the circumference of the deflated pumping chamber, is dependent upon the amount of non-contracting space between the tubes. However, in practice, it is very difficult to inflate a sheet of tubes, joined only at a tangent, without inducing high stress in the tubes or in an encircling sheath. Maintenance of the tubes in close proximity at high pressure necessitates some non-contracting space between the tubes. Furthermore, since the pumping chamber is meant to fit snugly to the heart, allocation must be made for fitting the pumping chamber to the heart in situ. Consequently, the tubes must be spaced apart for this purpose. As a result, the ejection volumes produced by the heart as a result of the spacing apart of the tubes in these conventional devices are significantly less than desired. This drawback occurs even if the inflated portion of the pumping chamber""s circumference were to produce its theoretical maximum of 36% diametric contraction.
What is needed, therefore, is a non-blood contacting ventricular assist device that generates a contraction, which exceeds the theoretical limit of conventional contractile balloon wraps, and hence, generates a greater maximum stroke volume. The device should not encumber the natural function of the heart and, in the event of failure, the device should not interfere with the natural pumping action of the heart. The possibility of further injury to the heart and adjacent vessels should also be minimized by providing gentle and physiologically correct pumping action. The device should not damage adjacent tissue or traumatize adjacent organs by compression or excessive localized temperatures. The ventricular assist device should also be configurable to assist the left, right or both ventricles.
In addition to cardiac assist devices which actively assist the heart in pumping blood (so-called xe2x80x9cactive devicesxe2x80x9d), the present invention also pertains to another type of heart assist device known as a xe2x80x9cpassive constraintxe2x80x9d or simply a xe2x80x9cpassivexe2x80x9d device. Passive devices serve to prevent cardiac expansion beyond a predetermined volumetric limit in patients suffering from cardiac dilation, hypertrophy and related conditions. In the absence of such constraint, the weakened heart muscle will lose its ability to pump blood and, in many cases, result in damage to the patient""s heart valves. In passive devices, the goal is not to augment or replace the natural heart""s pumping action but rather to assist the heart by applying a constraining force during the heart""s expansion (diastolic) phase.
Ideally, a passive device wrapped around the heart should mimic the natural resistance of the heart muscle itself to over-expansion. A healthy natural heart will exhibit a characteristic relationship between ventricular pressure and volume, such that small amounts of pressure will initially result in a desired expansion of the ventricular volume. During activity or exercise, the ventricles must also response to higher pressures to accommodate a greater volumetric expansion and, thereby, permit increased ventricular output. However, to achieve a desired ventricular output, especially during normal activity or exercise, the maximum ventricular end-diastole volume must still be constrained or else the ventricle, during contraction, will be unable to eject the necessary quantity of blood.
Unfortunately, conventional passive devices exhibit constraining forces that typically are not well matched to the natural physiology. Even when such passive wraps are constructed from elastic materials that respond to increases in pressure in accordance with Young""s law, the performance of such devices degrades over time, largely due to the in-growth of epicardial and/or interstitial cells within and around the device. This in-growth prevents the elastic elements of the device from stretching.
Thus, what is also needed is a passive cardiac device that can better mimic the natural heart""s response to increases in diastolic pressure and, in particular, devices that can continue to function and respond to such pressures despite in-growth of cells over time.
Methods and apparatus are disclosed for providing assistance to the ventricles of a natural heart. In one aspect of the invention, ventricular assist devices, capable of encircling at least a portion of the heart, are disclosed having multiple layers of inflatable elements. Such multi-layer devices induce contractions that overcome the limits of conventional contractile balloon wraps, and hence, generate a greater maximum stroke volume. Pumping modules incorporating multiple layers of inflatable elements are disclosed to wrap around, or attach to, one or both ventricles of a natural heart, or to any-other blood containing structure that enables natural circulation. The invention also encompasses a unified system that integrates the pumping modules of the present invention with other major components required for mechanical heart massage into one system that is completely implantable into the human thorax.
Thus, a unified system according to the present invention can be composed of a highly efficient pumping module (described in more detail below), one or more reservoirs of fluid and a control module. The control module can include an internal electronic controller for generating a suitably shaped pressure wave to be synchronized with the natural contraction of the heart, pumps, valves and/or regulators for delivering a pressurized fluid to the pumping module and a suitable power supply. In one embodiment, the power supply can include a transcutaneous energy-receiving device, and/or an implantable battery for storage of the received energy. The control module can further include a data transceiver.
The pumping module can also include conduits for distributing the pressurized fluid to the various inflatable elements of the pumping unit and attachment elements for attaching the pumping unit to the heart. The attachment mechanism can include direct attachment elements or tethering devices intended to prevent the wrap from slipping off the heart.
In accordance with another aspect of the invention, an easily attachable (and in at least some instances, a readily detachable) pumping unit is disclosed that is constructed of thin, collapsible, non-distensible, biocompatible material, which encases a multi-layer arrangement of inflatable elements. The inflatable elements can also be bound by a sheath that holds them in a defined geometry. For example, in one arrangement, the encircling sheath can bind sets of two or more inflatable elements in individual pockets, such that when the sheath is wrapped about a heart, the inflatable elements of each pocket will be stacked or juxtaposed along a radial line. The sheath also serves to join the sets of inflatable elements to each other along a line perpendicular to said radial line. This second dimension thus forms a circumferential restriction when placed around the heart.
The inflation elements can be tapered at one or both ends, and the resulting wrap curved in a plane, so that when joined end-to-end to form a continuous band, the wrap describes approximately the surface of a paraboloid of revolution. In this configuration, the surface of wrap that faces the epicardium of the heart presents a plurality of pockets each of which contain multiple layers of inflatable elements. The inflatable elements can be filled at time of implantation to conform the wrap to the heart. In one embodiment, particularly useful in active devices that are intended to assist the natural heart""s pumping action, the inflatable elements are filled with a flexible, deformable substance that substantially maintains its volume when compressed.
Unified systems according to the invention can further include one or more electrodes for implantation on the heart or at a suitable adjacent site (e.g., on the heart assist device), to sense the heart""s electrical signals and synchronize pump activation with the heart""s cycle. For example, the sensor can detect and/or monitor well-known EKG components such as the p-wave, or the q-r-s-wave (indicating the beginning of systole) and/or the t-wave (at the end of systole). The signals from such sensor electrodes are then used by an electronic controller to synchronize the release of actuating fluid to the pumping unit and, subsequently, to synchronize the evacuation of fluid from the pumping unit.
The unified system can further include an energy converter (e.g., a pump) and at least one plenum for storage of a pressurized volume of fluid of sufficient size to provide a flow at nearly constant pressure during systole and to provide a flow away from the heart assist device at nearly constant vacuum (i.e., at a pressure less than ambient) during diastole.
Fluid control can be accomplished in at least two different ways. In one embodiment, a single plenum is used to store the inflation fluid and a bi-directional constant pressure pump is used to both inflate and evacuate the heart assist device. The pump can be a electro-mechanical energy converter or a device that induced the patient""s own skeletal muscles to power a pump, or it can be a hybrid of both. Alternatively two uni-directional pumps can be used in tandem to fill and empty the inflatable elements of the heart assist device.
In a second embodiment, two plenums can be employed. One plenum is provided for storage of evacuated fluid and is preferably maintained at a sufficient state of evacuation so as to provide evacuating flow at a nearly constant pressure during the evacuation interval. A second plenum is provided for filling the inflatable elements of the heart assist device and is preferably maintained at a sufficient state of pressurization so as to fill the heart-pumping unit at a nearly constant pressure during systole. The unified system can further include a mechanical device or energy converter for continuously pumping fluid from the evacuated plenum to the pressurized plenum. The system can further include a controller or regulators to maintain the plenums at their respective pressurized states. In one embodiment, the energy converter, controller and plenums are contained in a single housing, the back of which has a convex surface curvature compatible with the internal abdominal cavity.
The system can further comprise a mechanical device or energy converter for continuously pumping fluid from the evacuated plenum to the low pressurize plenum and maintaining their respective pressurized states, and a second energy converter for continuously pumping fluid from the evacuated plenum to the high pressure plenum while maintaining their respective states. The two energy converters can pump substantially different flows, with the high pressure energy converter pumping a substantially lower flow. The system can further include a housing for containing the plenums and energy converters, the back of which can have a convex surface curvature compatible with the internal abdominal cavity.
Control systems in accordance with another aspect of the present invention can be electrically coupled to one or more electrodes that sense the heart""s electrical activity, and can further comprise an electronic controller for synchronized release of actuating fluid to the pumping unit for subsequent synchronized evacuation of fluid from the pumping unit. The system can also include a plenum for storage of a non-pressurized volume of fluid of sufficient size to provide a flow at nearly constant pressure during the release interval, said plenum used for storage of fluid, a mechanical device or energy converter for periodically pumping fluid from the storage plenum to the pumping unit and thus attaining a pressurized state in the pumping unit, said energy converter pumping toward the pumping unit during systole and pumping from the pumping unit during diastole, and a housing for containing the plenums and energy converter, the back of which has a convex surface curvature compatible with the internal abdominal cavity.
In an alternative system according to the present invention, the unified control system can also include: an internal electronic controller for receiving both AC and DC supply voltages, an external communication channel data stream and generating an actuating signal for the release of pressurized fluid, communication channel data stream and internal battery recharging signals, an actuating means for converting said actuating signal into a periodic movement of the valve member(s), a pumping unit having an associated attachment means wherein the unit is attached directly to the heart or tethered to sites near the heart, said sites providing a restoring force directed from the apex to the base so as to counter the forces applied to the wrap by the heart, a volume displacement chamber containing the energy converter and plenums, a hermetic coupling means for connecting said controller to said energy converter, said communication channel data streams and internal battery recharging signal; a detecting means for generating said actuating signal in response to an electrically derived signal from the heart and/or measurement of flows, pressures, tensions related to the heart""s ventricles for generating said actuating signal, and a housing the back of which has a surface curvature compatible with the abdominal cavity for containing said electronic controller, said actuating means, said blood pumping unit, said hermetic coupling means and said detecting means connected so as to form a unified system.
In accordance with another aspect of the present invention, the unified system can include a rechargeable internal battery for subcutaneous implantation to supply an internal DC supply voltage. The system can further include an external battery for providing a DC voltage to an external controller; with the external controller converting DC voltage received from the external battery and/or external power supply to AC voltage for transfer by a subcutaneous energy transformer to power the unified system and/or for recharging external the internal battery. The system can also include a computer interface for connecting said device to a computer for control and monitoring of the device; a display means for control status and alarm display; a transcutaneous energy transformer for transmitting said AC voltage across the skin; a transcutaneous information telemetry unit for bi-directional transmitting said communication channel data streams between said external controller and the implanted components of the system; as well as a connector for connecting said internal battery to said internal controller and an in-line connector for connecting said transcutaneous energy transformer to the internal battery.
In accordance with another aspect of the present invention, the unified system can include an implantable stimulator to supply an internal DC stimulus voltage and a rechargeable internal battery controlled by an internal controller for actuating a selected patient""s skeletal muscle with the muscle forming a component of a mechanical pressurizing system. This system can be electrically actuated so as to maintain a desired steady-state hydraulic fluid pressure so that pressurized fluid can be held in reserve in a plenum for transfer to a pumping unit, said pumping unit powered by the pressurized fluid reserve, and said mechanical pressurizing system providing for a evacuated side, this side attached to a second plenum which actively draws fluid from the pumping unit during diastole.
In accordance with another aspect of the present invention, the rechargeable internal battery can be controlled by the internal controller for actuating a selected patient""s skeletal muscle; with the muscle forming a component of a mechanical pressurizing and electrical energy generating system. This system can be actuated so as to maintain a desired steady-state hydraulic fluid pressure so that pressurized fluid can be held in reserve in a plenum for transfer to the pumping unit via an electrically generating element. The pumping unit can thus be powered by the pressurized fluid reserve, with this mechanical pressurizing system providing for a evacuated side, which can be attached to a second plenum that actively draws fluid from the pumping unit during diastole. The pressurized plenum can further be fitted with an electrically driven valve on the output side and the evacuated plenum fitted with an electrically actuated valve on the input side, and the valves can be powered by the internal battery and controlled by the internal controller. The system can further include an optional external computer interface for connecting said device to a computer for control and monitoring of the device; a display means for control status and alarm display; a transcutaneous information telemetry unit for bi-directional transmitting said communication channel data streams between said external controller and the unified system; a connector for connecting said internal battery to said internal controller and an in-line connector for connecting said telemetry to said internal controller.
The control systems of the present invention can be totally implantable and require no physical connections through the skin to the outside. The system can be powered by an implantable battery or directly by a transcutaneous energy transfer system. The battery can be recharged by the transcutaneous energy transfer system or by a muscle powered device. The device can be controlled and monitored remotely via a transcutaneous information telemetry or by said internal electronic controller.
In addition, the device of the present invention can be adapted to pneumatic as well as hydraulic activation, and alternative energy sources may be utilized such as a Stirling type engine, osmotic pressure vessel, muscle powered generator or a nuclear thermal source.
Anatomical fit to the heart and the maximum contraction length of the wrap are important factors in the clinical success of non-blood contacting, volume-amplified inflatable cardiac assist devices. In addition to the contraction limitations of the single layer geometry, prior art devices typically also exhibit poor anatomical compliance (xe2x80x9cfitxe2x80x9d). In the present invention a superior fit is achieved by providing a multiple layer geometry with a contraction length large enough to provide adequate non-contracting regions for attachment and gathering.
Furthermore, the inflation elements are shaped so as to minimize the size of the non-contracting regions. Attaching a pocket between the pumping unit and natural heart can make refined fit adjustments. This pocket can be filled with a hydrogel in situ. The gel can form a thin protective and close fitting layer between the pumping unit and natural heart. The gel can also be constrained entirely within the pocket or allowed to permeate the pocket so as to form a bond between the epicardium of the natural heart and the pumping unit.
In one preferred embodiment, the major components of the invention are integrated so that their individual functions are complementary. The specifications on these individual components are representative of a unified solution to the problems outlined above and may be unique to the arrangement disclosed. They reflect not only the actuation of a physiologically acceptable contraction of the heart resulting in enhance blood flow through the heart, but also relate to issues of maintenance, failure modes, reliability, energy economy, biological compatibility, and quality of life.
Heart assist devices are disclosed to provide an arrangement of inflation chambers or tubes that, when placed in juxtaposition (e.g., forming concentric layers of tubes), generate a contraction which is substantially greater than 36% of the uninflated length or circumference.
In another preferred embodiment, the inflation chambers can be oriented so that their axes are oriented along, but not necessarily parallel to the major (longitudinal or vertical) axis of the natural heart. By employing elongate tubes that are generally aligned with longitudinal axis of the heart, efficiency of pumping is further enhanced because the contractile dimension of the device is aligned with the natural (circumferentially inward) contractile direction of the heart.
The present invention also provides a method of positioning and layering the inflation chambers, and an arrangement of non-contracting regions (accounting for as much as 40% of the wrap""s circumference) which allows for in situ fit adjustments while maintaining a contraction of at least 36% over the entire length of the wrap. The present invention further provides specifications for shape and dimension of the inflation chambers, and control of inflation that results in a gentle, effective, and physiologically correct contraction when the wrap is coupled to the heart.
In another aspect of the invention, improved designs and structures for heart-contacting assistance devices are disclosed. For example, the pumping modules can further include a thin, flexible liner between the heart and the inflation chambers to pad the heart. Preferably, the liner is filled with a substance, such as a hydrogel material, which changes the shape, and not substantially the volume, of the liner when compressed. This liner provides a customized fit to the natural heart.
In another aspect, the invention also provides a ventricular assist device system which can wrap around the natural heart and assist the natural heart in pumping without coming into contact with blood, and further provides a device that mimics the pumping action of the natural heart and is not directly coupled to the heart, so that in the event of failure, the device does not interfere with the natural pumping action of the heart.
The present invention also provides a ventricular assist device that occupies a volume that is less than the volume of the ejected blood. Likewise, methods are disclosed in which the function of the heart muscle is assisted by generating an encircling contraction around the heart which exceeds the theoretical limit of single layer balloon wraps, and hence, generates a greater maximum stroke volume.
The pumping devices of the present invention can be fixed with respect to the human heart, but not necessarily directly attached to it, and all of which is free to move with respect to other organs or bones. Moreover, the devices can be configured suitably for use as a right, left, or bi-ventricular assist devices.
The invention can further encompass ventricular assist systems with a portion of the control electronics-implanted within the patient and the remainder of the control electronics provided on a small portable unit to be worn on a belt or other clothing or externally attached to the patient""s skin.
In yet another aspect of the invention, multi-layered balloon wraps can be used in passive assistance devices to provide structures that restrain cardiac hypertrophy and mimic the natural resistance of the heart tissue to over-expansion. By choosing an appropriate inflation pressure for the balloon elements and then sealing them, the fluid pressure within the balloons can provide a resistance analogous to the Frank-Starling effect exhibited by cardiac tissue.
In such passive systems, if the heart continues to dilate, the balloons will flatten more to accommodate the enlargement but the pressure applied to the heart by the balloons will be greater. Moreover, unlike mesh-type passive girdles, which rely upon an open structure to accommodate the expansion and contraction of the heart, the multi-layer inflatable structures of present invention permit the use of solid wrap devices, which are less likely to loss their effectiveness over time due to tissue in-growth.
In addition, the passive devices of the present invention can be adjusted. For example, if the heart shrinks, the balloon elements can be periodically filled to a greater extent in order to tighten the wrap and the pressure applied by the partially inflated balloons will be less.
The invention can also provide architecture that can be adapted to many different geometrical configurations to meet the requirements of different actuating techniques within the overall constraints of the invention.